Cell, Vol. 15, 421-428, October 1978, Copyright 8 1978 by MIT Coordinate Synthesis of Two Myosins in Wild-Type and Mutant Nematode Muscle during Larval Development Robert L. Garcea, Fred Schachat and Henry F. Epstein Department of Pharmacology Stanford University School of Medicine Stanford, California 94305 Summary In this paper we examine the role of two myosins in body-wall muscle cells of the nematode Cae- norhabditis elegans. Large populations of nema- todes are synchronized, and the synthesis and accumulation of myosin heavy chains and total protein are followed through postmitotic larval development. Growth is exponential with time for both the wild-type N2 and the body-wall muscle- defective mutant E675, with a longer doubling time for the mutant. Utilizing the electrophoretic polymorphism of the E675 myosin heavy chains, we show that distinguishable classes of heavy chains accumulate differentially throughout de- velopment. lmmunochemical measurements con- firm a similar result in N2. Total myosin heavy chain accumulation is also quantitatively similar for the two strains. Myosin heavy chain relative synthetic rates as determined by pulse-labeling are constant throughout development and are equivalent for the two strains. The final fraction of accumulated uric-54 to total heavy chains of ap- proximately 0.63 equals the constant synthetic fraction of approximately 0.62. Since myosin heavy chain accumulation and relative synthesis are equivalent, we conclude that the turnover of heavy chains is also similar in N2 and E675 despite the extensive structural and functional disruption within body-wall muscle cells of the latter strain. Since the accumulated fraction of uric-54 myosin heavy chains reaches a plateau at the constant synthetic fraction, myosin accumulation in the body-wall muscle cells may be attributed to a constant ratio of synthetic rates of the two body-wall myosin species. The coordi- nate synthesis of two myosins in the same body- wall muscle cells is discussed. Introduction The assembly of the muscle filament lattice is a complex example of biological structure formation which involves maintaining proper stoichiometry of the components being assembled and directing the specificity of their interaction to achieve a very regular structure. The influence of the final struc- ture on the further synthesis of components is also an issue; in particular, the discovery of two differ- ent myosin heavy chains within the same muscle type by Starr and Offer (1973) and Epstein, Water- ston and Brenner (1974) raises the specific ques- tion of their synthetic control during filament as- sembly. Larval development in the nematode Cae- norhabditis elegans is accompanied by a dramatic increase in the assembly of new body-wall sarco- meres (Mackenzie et al., 1978b), and thus by fol- lowing the developmental course of myosin synthe- sis and accumulation, regulatory interactions be- tween the myosins and other components of the assembling filament lattice might be determined. In a similar manner, the synthetic control of con- tractile proteins has been studied during vertebrate skeletal muscle development in vitro (Paterson and Strohman, 1972; Whalen, Butler-Browne and Gros, 1976; Devlin and Emerson, 1978). The body-wall muscle cells of C. elegans are a model system useful for this study because the larval develop- mental period is short (Byerly, Cassada and Rus- sell, 1976), the number of muscle cells remains constant (Sulston and Horvitz, 1977), the muscle proteins comprise a significant proportion of total protein (Harris and Epstein, 1977) and mutants are available that affect specific myosin heavy chains and disrupt the filament lattice (Epstein et al., 1974). C. elegans has several distinct muscle cell types: body-wall, pharynx, vulva, anal sphincter and uterus. The body-wall musculature is by far the largest set in terms of mass and number, com- prised of 95 cells arranged in four longitudinal bundles (White et al., 1976). The uric-54 genetic locus in C. elegans specifies one of the classes of myosin heavy chains present in the body-wall mus- cle (Epstein et al., 1974). The uric-54 mutant E675 has a partially deleted heavy chain (MacLeod, Wa- terston and Brenner, 1977a), reducing its mass from 210,000 daltons (210 kd) to 203 kd, whereas the El90 allele is apparently null for this type of myosin heavy chain (Epstein, Schachat and Wolff, 1977; Schachat, Harris and Epstein, 1977; Mac- Leod et al., 1977b). Both E675 and El90 have severely disrupted body-wall muscle structure and decreased motility throughout larval development. The sodium dodecylsulfate-polyacrylamide gel electrophoretic (SDS-PAGE) pattern of myosin heavy chains reveals 210, 206 and 203 kd species while the wild-type N2 has only 210 and 206 kd bands. Three types of evidence have defined the anatomic location of the myosin heavy chains spe- cies. Dissection of E675 shows that the 203 and 210 kd species are associated with the body-wall (Epstein et al., 1974); residual thick filament-like structures in El90 body-wall muscle suggest that some of the non-uric-54 210 kd species may be present (Epstein et al., 1974; MacLeod et al., 1977b); immunocytochemical localization using